1. Field of the Invention
The present invention relates to a high temperature, solid electrolyte, elongated, electrochemical cell containing a plurality of annular, self-supporting cell segments or elements contacting annular interconnection members disposed between the cell elements; and to a plurality of such cells electronically interconnected in a generator chamber in an electrochemical generator apparatus.
2. Description of the Prior Art
Fuel cells having a concentric configuration of tubular electrodes, supported and spaced away from each other, are well known in the art, and taught, for example, by Fay et al., in U.S. Pat. No. 3,259,524. There, a noble metal impregnated, closed-tube, outer graphite electrode was supported in a spaced relationship in a housing, by inert plugs, and contained a spaced apart, closed-tube, inner palladium electrode into the bottom of which a fuel was fed by means of a long fuel tube. The electrolyte used was an aqueous solution of potassium hydroxide, and the apparatus operated at a low temperature. It was later found, that solid electrolyte fuel cells, operating at a high temperature, provided a much more efficient electrochemical apparatus. Electrical connections between adjacent cells posed interesting problems in these solid oxide type apparatus.
White, in U.S. Pat. Nos. 3,402,230, and 3,460,991, taught a self-supporting, one piece, tubular, high temperature, solid electrolyte fuel cell tube. There, an elongated, tubular, gas tight cell stack was taught, with solid electrolyte generally disposed between air electrodes and fuel electrodes. The cell tube was unitarily formed as a continuous tube, rather than assembled as a series of individual cells. Gaps between the cells were filled with an overlap of top air electrode overrunning the underlying solid electrolyte to physically and electrically contact the bottom electrode forming an electrode-to-electrode connection on the tubular structure. Calcia stabilized electrolyte was taught, along with a variety of cathode and anode materials. Electronic connections between individual fuel cell stack tubes were made in series, directly from the inner electrode of one cell stack tube to the outer electrode of a parallel, adjacent cell stack tube.
Tannenberger et al., in U.S. Pat. No. 3,525,646, also taught a tubular, solid oxide, fuel cell stack structure, similar to that taught by White, but, supported on a porous tube, and having segmentation between top electrode layers and between bottom electrode layers, but electronic connection between bottom and top electrodes along its length. Archer et al., U.S. Pat. No. 3,526,549 also dealt with flat, solid electrolyte in cylindrical cell configuration, utilizing a fuel and an oxidant. There, fuel and air electrodes were coated on opposite faces of a solid electrolyte disc, between contacting annular gaskets and space apart annular current collectors which defined air and fuel compartments. Both air and fuel were fed through manifolds internal to the cells, where air and fuel flow across electrodes was 90 to the axial length of the cylindrical configuration.
Fally et al., in U.S. Pat. No. 3,668,010, taught solid electrolyte cell stacks in tubular form, having electrodes on their inner and outer surfaces, where a round metal plug was used to fill holes made through the electrolyte tube, so as to electronically connect the outer electrode of one cell with the inner electrode of an adjacent cell in series, similarly to White, at a number of places around the circumference of the tube. Schmidberger, in U.S. Pat. No. 4,174,260, taught internal, tubular interconnection rings, for series-connected cell stacks, where the inner electrode of a tubular, solid oxide, fuel cell arrangement overlapped an inner portion of the interconnection ring, and the outer electrode overlapped an outer portion of the interconnection ring, so as to electronically connect inner and outer electrodes, again, similarly to White.
More to date, Isenberg, in U.S. Pat. No. 4,395,468, taught a fuel cell arrangement incorporating long, thin, continuous, tubular, solid oxide fuel cells, where air was fed into the center of the single cell tube, which contained an air electrode on a support tube, by means of air feed tubes, which could be inserted into the porous support tube for air distribution. Fuel flowed between and around the outer fuel electrodes of parallel sets of single fuel cells. A single air inlet, fuel inlet, and reaction product combustion outlet were taught. Isenberg, in U.S. Pat. No. 4,490,444, taught solid oxide fuel cell configurations and interconnections. The fuel cell design was a continuous design used by Isenberg in U.S. Pat. No. 4,395,468, and was constructed with a long, separate, central, porous support tube covered by an air electrode, solid electrolyte and fuel electrode. Each fuel cell had a long, single interconnection, and an attached metal felt strip, extending the length of the fuel cell, for the air electrode of one cell to electronically connect to the fuel electrode of adjacent cell. Cells could also be connected in parallel by means of additional, attached metal felt strips.
An alternate design taught by Isenberg in U.S. Pat. No. 4,490,444, involved circumferential segmentation of the elongated single cells. By dividing the long cell into segments, each segment would be contacted with similarly depleted air and fuel, rather than one end of a long cell seeing fresh air and fresh fuel and the other end seeing
depleted air and depleted fuel. The circumferential portion between segments was taught as being an electronically insulating solid electrolyte material, such as yttria stabilized zirconia. In all cases, the air electrode could be made of doped or undoped oxides or mixtures of oxides in the pervoskite family, such as LaMnO.sub.3, and the fuel electrode could be made of a nickel zirconia cermet material. Grimble et al. further advanced the Isenberg design with U.S. Ser. No. 852,865, filed on Apr. 16, 1986, and assigned to the assignee of this invention, by in-situ reforming of fuel along the entire length of the fuel cell, and by adding separate fuel feed tubes exterior to and disposed between the fuel cells to provide unreformed fuel feed.
Ackerman et al., in U.S. Pat. No. 4,476,198, departed from spaced apart tubular cells, teaching solid oxide fuel cells arranged in a close packed, contacting array in a generator. Here, a corrugated plate structure of fuel electrode, air electrode and electrolyte formed opposing triangular channels in contact with an electrical interconnection material layer disposed between those corrugated layers and connecting positive and negative electrodes of different layers. All the layers were in bonded series-connected electronic contact through the interconnection. Oxidant was fed into a triangular air electrode space by means of air feed tubes extending the length of the air electrode space. Single oxidant inlet, fuel inlet, generator, and reaction product combustion outlet chambers were taught. The fuel and air electrodes and electrolyte disposed between them were of a long, continuous design.
The fuel cell and generator of U.S. Pat. Nos. 4,395,468 and 4,490,444, the most current of the tubular designs, relate to long cells, usually of about 15 mm outside diameter, where the diameter of the cell cannot be increased, however, not without also raising the cell resistance. These fuel cells have associated power losses due to the resistance of the oxide air electrodes mainly and due to the fact that the electrical current in the air electrode flows in a circumferential pattern to an axially positioned interconnection strip, and from there to a metallic current collector felt. These cells also require time consuming and expensive masking-demasking steps in the deposition processes used and add substantially to cell cost, which cost becomes very significant because of the large number of cells required in each generator. Also, cell designs of White, Tannenberger et al., Schmidberger, and similar series-connected stack type processes, require labor intensive masking-demasking steps and electrical interconnection steps adding substantially to design complication and cell cost.
Present cells of this tubular configuration can achieve moderate power levels, about 20 watts per 30 cm long fuel cell tube. Power can be increased by extending the length of the active cell, however, cell lengths over about 1.2 meters (4 feet) present fabrication problems. Also, long and small bore cell tubes present a problem in air manifolding in generators, due to increased pressure drop. While the present design is reliable and quite adequate for small and medium size generators, a new design of both generator and fuel cell is needed, which would provide major cost reductions in cell fabrication, while at the same time providing higher generator power levels.